Black holes are arguably the most mysterious and captivating objects in the universe. Personally, I dream of studying them at university. They have been featured in many science fiction books and movies, such as Nolan’s Interstellar. But how do they really look like? What rules govern them and how do they form? I’ll try to answer some of these questions in this post.
What is a black hole?
A black hole is a region of space from which nothing, even light, can escape. This means that we can’t see them directly. We can detect black holes by looking at the stars around them. In April 2019, scientists at ETH did just that and unveiled the first ever image of a black hole. Of course, we already predicted them in the 1960s, as they seemed to naturally follow from general relativity.
Black holes generate tremendous gravitational fields. You may be asking, how can gravity attract light, since photons are massless? Newton’s equation does require mass to be present in order to feel the force of gravity, but general relativity does not. Gravitational deflection of light is something that even our sun does, and it has been experimentally tested many times. These gravitational lenses can allow us to see stars that are behind our sun. I’ll probably make a separate post on this topic in the future.
At the center of a black hole there is a singularity. It is a region of infinite density, where the curvature of spacetime becomes infinite. It has zero volume and it is a place where laws of general relativity break down. Still not a lot is known about this region. We might need a solid theory of quantum gravity in order to understand what really happens.
How do they form?
Black holes form from the gravitational collapse of huge stars. Technically, everything can be a black hole if it is packed tightly enough. The size required for a body to undergo irreversible collapse is called the Schwarzschild radius. It’s the tipping point for black holes.
Most stars that form black holes are at least 5 times the mass of the Sun. When they collapse, some larger ones explode as supernovas. These gorgeous explosions are the reason for the prevalence of some elements in the universe and definitely deserve an article of their own.
There are many ways to characterise black holes. Firstly, black holes nomen omen act as ideal blackbodies. So one of the ways of characterising them is through temperature. Stephen Hawking writes about this extensively in A Brief History of Time.
Black holes are extremely cold on the inside, but very hot just outside the event horizon. This is because the matter falling into the black hole gets accelerated to near the speed of light, so the particles have huge average kinetic energy. You might be thinking that the inside of the black hole should have no temperature at all, since no electromagnetic radiation can escape it. However, there is Hawking radiation to consider, which I’ll talk about in another post. It arises due to quantum effects near the event horizon. Pretty cool how quantum effects can affect huge cosmic bodies.
Another way to characterise them is through radius. We have already seen the equation for the Schwarzschild radius, but let’s try to get a sense of scale. The radius for our Sun is around 3km. This means that the Sun would have to collapse to the size of a small town in order to become a black hole. What’s interesting is that if the Sun became a black hole right now, we wouldn’t feel a big difference gravitationally.
The largest known black hole has a radius of 1300 AU. That is 1300 times the distance from the Earth to the Sun. Scary. But black holes could be more familiar than it appears. A very probable theory is that the gravitational mass in the center of the Milky Way is a supermassive black hole. Those have a radius up to 400 AU. For a human to become a black hole, they would need to shrink to 1.04×10−25 m in radius. That is way smaller than the radius of a proton, which is about 10−15 m.
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NASA. (2019, April 19). How scientists captured the first image of a black hole – teachable moments. NASA. https://www.jpl.nasa.gov/edu/news/2019/4/19/how-scientists-captured-the-first-image-of-a-black-hole/.
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